How to bind Height of element in parent Page to Height of ChildWindow

[fjparisIII]

I want to bind the Height of a ChildWindow to the Height of an element in the Page that shows the ChildWindow. The only descriptions I've seen are bindings between elements in the same Control. Is what I want to do even possible, and if so, what is the syntax of the Binding expression that would be in the ChildWindow? I first tried to set the Height of the ChildWindow to the Height of the element in the Page immediately after constructing the ChildWindow and before calling ChildWindow.Show, but for some mysterious reason, this positions the top of the ChildWindow above the top of the Page, which is unacceptable. Naturally I want to keep the ChildWindow centered on the page, or at least centered on a specific element within the Page.